Electric drive machines are quickly replacing mechanical drive machines both in on-highway and off-highway applications. An electric drive machine consists generally of an engine drivingly coupled to a generator. As a mixture of fuel and air is burned within the engine, a mechanical rotation is created that drives the generator to produce electric power. The electric power is sent to a motor or series of motors associated with traction devices of the machine to propel the machine.
Ideally, the engine drives the generator with a relatively constant torque and speed, and the generator accordingly produces a corresponding electrical output that is passed onto the traction motors. The traction motors are driven by the electric power from the generator to rotate at an operator selected speed and with a torque that varies with machine loading. A relatively constant engine torque and speed should result in relatively low fuel consumption and relatively smooth machine operation. However, the varying load on the traction motor can be affected by external factors that are often unpredictable and cannot always be controlled. And changes in torque loading of the motor can affect operation of the generator, which can translate into undesirable fluctuations in engine performance.
For example, when an external load is suddenly applied to the traction devices of the machine, the motor will draw extra power from the generator to maintain the operator-desired speed of the traction devices. This extra power drawn from the motor will load the generator, and the generator will attempt to provide for the increase in electric power demand by drawing more mechanical power from the engine and converting the additional mechanical power to electric power. To provide for the extra mechanical power, the engine must draw in extra combustion air and extra fuel. Similarly, when an electrical load is suddenly removed from the generator by the motor, the generator will quickly reduce its electric power production by drawing less mechanical power from the engine and the engine must respond by drawing in less combustion air and fuel.
Although the motor and generator may respond quickly to the changes in load, the engine may have a much slower response time. As a result of an increased mechanical load from the generator and due to the slower response of the engine, the engine may lug (i.e., the engine may slow as a torque load increases) until the additional fuel and air can be directed into the engine and the engine can begin producing the higher output of mechanical power required by the generator. Similarly, as a result of a decreased mechanical load and because of the slower response of the engine, the engine may overspeed until the fuel and air directed into the engine can be reduced. Engine lugging or overspeeding can cause machine performance to fluctuate undesirably.
Historically, attempts to smooth fluctuations in the performance characteristics of a machine having an electric drive have included feedforward fueling of the engine. Specifically, if the change in electric load applied to the motor can be sensed soon enough after its application, a fueling command indicative of an impending mechanical load change can be directed to the engine before that mechanical load change can cause the engine to operate undesirably. In this manner, the engine can be given time to respond to the impending mechanical load change prior to the mechanical load on the engine actually changing. This forewarning may help reduce a magnitude of engine lugging or overspeeding as a result of the mechanical load change.
One attempt to provide feedforward control is disclosed in U.S. Pat. No. 7,098,628 (the '628 patent) issued to Maehara et al. on Aug. 29, 2006. In particular, the '628 patent discloses a generator control system for a vehicle that includes an AC generator driven by an engine, a load current detector, a driving-torque-increase calculator, a field current control means, and an engine power adjusting means. During operation, the driving-torque-increase calculator calculates a predicted increase in driving torque required from the engine by the AC generator to provide for an increase in the current supplied to an electric load as detected by the load current detector. When the predicted increase in driving torque is greater than a predetermined value, the engine power adjusting means adjusts engine power according to the predicted increase. While engine power is being adjusted, the field current control means limits an increase rate of the generator's field current within a predetermined value. In one embodiment, the field current is limited until the engine attains a predetermined speed at the increased driving torque. In another embodiment, the field current is limited until a preset time passes after the engine power is adjusted. By limiting the field current during adjustment of engine power, the likelihood of engine lugging or overspeeding may be reduced.
Although the '628 patent may help reduce the likelihood of engine lugging or overspeeding, it may still be problematic. Specifically, because the field current is limited during the engine power adjustment, the electric power provided by the generator to an associated drive motor may be less than desired and result in an unresponsive machine. And because the engine power adjustment does not commence until after the change in electric load has already been applied to the generator, the duration of the less-than-desired performance may be substantial. Finally, because the generator control system only limits an increase rate of the generator's field current when the increase is greater than the predetermined value, the system may be prone to dithering performance in response to small changes (e.g., noise and short transient torque fluctuations) in loading at the motor.
The disclosed drivetrain system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.